In the 1970s, French dentist Dr. Duret introduced computer-aided design and computer-aided manufacturing (CAD/CAM) into the dental field. The principle of this technology as described in U.S. Pat. No. 4,742,464 is: “optical impression” is formed by collecting the three-dimensional reconstruction of tooth preparation via photoelectric devices; after inputting the optical impression into computer, “optical working model” is formed in the three-dimensional reconstruction software; by use of CAD software and careful adjustment, a “digital wax pattern” of the dental restoration is formed; finally, the data of the digital wax pattern as a control parameter is input to CNC milling machine, and then the pre-fixed blank is milled into the designed form; following the special treatment according to the characteristics of selected materials, the dental restoration with individual shape is finished. This invention reduces the reliance on the complicated manual techniques of manufacturing dental restorations which need to be accumulated over a long period of time, improves the accuracy and reliability of the dental restorations, and reduces the pains of patients brought by making impression of the wounded tissue. In recent years, with the rapid development of CAD/CAM technology and machinable restorative materials, especially the machinable ceramic materials, the development and the application of dental all-ceramic restorations are further promoted.
According to the different degree of density, machinable ceramic blanks can be divided into two categories. The one kind includes silicate-based glass and glass-ceramic, hard alumina and hard zirconia ceramic blanks. These prefabricated ceramic blanks are fixed onto the working table of CNC milling machine, and then the restorations are manufactured according to the digital wax pattern with equivalent ratio. The other kind includes porous alumina and zirconia ceramic blanks made by dry pressing and partially sintering. These prefabricated ceramic blanks are fixed onto the working table of CNC milling machine, and then the restoration bodies are manufactured according to the digital wax pattern with special enlarged ratio. After the second time sintering, the bodies become the final dense restorations. There are a number of patents related to the CAD/CAM manufacturing methods thereof, such as US20050261795 A1, EP0824897B1, CN201676029U, CN202682074U, CN202682075U, CN102579148A, et al.
In the CAD/CAM manufacturing methods thereof, in order to prevent restoration bodies from being gradually separated from the blanks with the reduction of the surrounding materials during processing, which could reduce the stability and precision of milling and even lead to the fracture of bodies, connecting bars (also called as support pillar, supporter, connector, tab, etc) on the outer surfaces of the bodies that connect to the surrounding blanks are necessary. After forming the dental restoration bodies, the technicians need to remove them from the residual blank manually by cutting the connecting bars, and then grinding the residual partial connecting bars and the surrounding area of connecting points on the outer surface of the bodies with a dental low-speed handpiece (as shown in FIG. 1). Manually grinding the connecting bars not only increases the technician's workload, reduces the advantages of automatic CAD/CAM technology, but also decreases the smoothness of the outer surface, which affects aesthetics, accelerates pigmentation and even discoloration on the surface, and causes excessive abrasion of adjacent teeth due to the rough surface. Therefore, grinding and polishing the outer surface of restorations is necessary after manually grinding of the connecting bars. Vibration during grinding may cause chipping on the thin parts of restorations, especially on the edge; and manually grinding introduces local stress and micro-defects to the surface of restoration, which reduces the long-term stability and reliability of the restorations, and even leads to premature fracture and restorative failure.
The reason of these drawbacks of the prior art is that both inner and outer surfaces of the dental restorations are formed simultaneously by milling the both surfaces of prefabricated blanks with multi-axis milling method. Thus, the use of connecting bars (also called as supporter, connecting bar, connector, linker, tab, etc.) is necessary. This is the common manufacturing method of these CAD/CAM technologies. As said in CN103479442A, before sintering the bodies of ceramic copings for both crown and bridge, connecting bars need to be manually cut off. As said in EP 2010061119 and CN 102695471A, 30 denture components are obtained after cutting off the tabs which connect the components with the solid blanks. The U.S. Pat. No. 8,141,217 B2 provides a prefabricated blank with large size. As FIG. 2 of this patent shows, connecting bars are used to connect the dental restoration bodies with the surrounding blank during processing. The patent US2009059154 invents a new kind of glass, glass-ceramic materials and dental components made therefrom. As FIG. 2b of this patent shows, connecting bars are used to connect the dental restoration bodies with the surrounding blank during processing. Therefore, grinding and polishing the outer surface of restorations is necessary after manually grinding of the connecting bars.
As described in CN102579148A and CN102302381A, zirconia restorations are separated from the ceramic blanks, and then the connecting bars on the outer surface are ground by a dental low-speed handpiece at the speed of 10000-20000 r/min. At the same time in order to improve the smoothness, the surface of the crown needs to be further ground.
Another common drawback of prior art is that the formed dental restorations can not be directly applied in clinic, and must be manually polished by technicians, and/or veneered, glazed before being applied. The reason of the above mentioned drawbacks is that the prefabricated ceramic blanks are dry. So the surface of the formed dental restorations are very rough, which not only affects aesthetics, but also leads to excessive abrasion of the opposite natural teeth during chewing (Oh W S, et al. Factors affecting enamel and ceramic wear: A literature review. J Prosthet Dent, 2002, 87 (4): 451-9; L. Wang, et al. Friction and wear behaviors of dental ceramics against natural tooth enamel, J Eur Ceram Soc, 2012, 32: 2599-2606). The Prefabricated ceramic blanks generally include unsintered green-bodies and partially sintered blanks. The unsintered green-bodies has plasticity, and a certain degree of plastic deformation can occur during processing to reduce the risk of accidental chipping. Ideally, the surface of dental restorations obtained by milling the green-bodies is smoother than that obtained by milling the partially sintered blanks. But since the plastic deformation is limited and the strength is insufficient of the complete dry green-bodies, they are difficult to sustain a large milling force and keep the integrity. This technique can not be realized. The mechanism of milling pre-sintered blanks is brittle delamination, because the granule sintering neck has been partially formed. The size and the form of debris are irregular and the milling process is not easy to be controlled. Thus the surface of dental restorations is very rough. However, due to the fact that the partially sintered ceramic blanks can resist a large milling force and the secondary sintering is relatively fast, the partially sintered ceramic blanks are normally used in prior art (Frank Thomas Filser. Direct Ceramic Machining of Ceramic Dental Restorations. 2001, p80-81; WO1999047065A1).
For reducing the negative effects of the rough surface of the formed dental restorations, dental technicians need to manually polish, glaze or veneer the restorations before application. All commonly used dry green body and partially sintered blanks have these drawbacks, no matter they are produced by dry pressing the ceramic powders or by wet-forming. As described in US20050261795A1, CN102579148A and CN102302381A, the fully sintered dental restorations need to be further polished or glazed.
Above all, the outer surface of the dental all-ceramic restorations produced by prior art is very rough, which must be polished, glazed or veneered to meet the requirements of surface smoothness. The dental all-ceramic restorations produced by prior art cannot obtain high smoothness and be directly used in clinical dental restorative treatment.